1. Technical Field
The invention relates to an electrophotographic photoreceptor for use in devices forming an image by an electrophotographic method, such as copying machine, and a process cartridge and an image-forming apparatus using the electrophotographic photoreceptor.
2. Related Art
Recently, the electrophotographic method has been used widely, for example, in copying machines, printers, and the like. An electrophotographic photoreceptor for use in image-forming apparatuses utilizing the electrophotographic method (hereinafter, sometimes referred to as a “photoreceptor”) comes into contact with various materials and is exposed to various stresses in the apparatus and thus deteriorates gradually. However, on the other hand, it should also cope with the demand for digitalization and colorization of image-forming apparatuses and high reliability.
For example, the process of charging the photoreceptor involves the following problems. First, in the non-contact charging mode, discharge products deposit on the photoreceptor, causing problems such as image blurring. For this reason, for example, a system in which particles functioning as a polishing agent are added to the developer and the developer is removed in a cleaning unit is used in some cases for removal of the discharge products deposited on the photoreceptor. In such a case, the surface of the photoreceptor deteriorates gradually due to abrasion. On the other hand, systems utilizing the contact charging mode have been used widely recently. In this mode as well, the abrasion of the photoreceptor may be accelerated.
Under the circumstances above, there exists a need for prolongation of the life of the electrophotographic photoreceptor. Prolongation of the life of electrophotographic photoreceptor requires improvement in abrasion resistance and thus an increase in the hardness of the photoreceptor surface.
However, in a photoreceptor made of amorphous silicon having a surface of high-hardness, discharge products and the like deposit thereon often causing image blurring and image deformation, and this phenomenon is more distinctive, especially under high-humidity conditions. The same is true for the surface layer of an organic photoreceptor having an organic photosensitive layer.
For prevention of the problems mentioned above, carbon-based materials are often used as the surface layer of the photoreceptor.
However, improvement in film hardness of a carbon-based film, such as a hydrogenated amorphous carbon film (a-C:H) or a fluorinated film thereof (a-C:H,F), leads to color development of the film. Thus, abrasion of the surface layer of the carbon-based film leads to an increase in light transmission efficiency of the surface layer over time, causing a problem of an increase in sensitivity of the photosensitive layer below the surface layer. In addition, uneven abrasion of the surface layer in the surface direction also leads to uneven distribution of the sensitivity of the photosensitive-layer, causing a problem of image irregularity, especially when a halftone image is formed.
On the other hand, improvement in hardness and improvement in transparency are known to have a trade off relationship, as a general characteristic of carbon-based thin film materials. This is because, as for the carbon bond in the film, it is necessary to increase the rate of diamond-type sp3 bonding for improvement in hardness, while these films inevitably have graphite-type sp2 bonding causing light absorption, and reduction in the rate of the graphite-type sp2 bonding in the film by hydrogenation or the like results in improvement in transparency but also deterioration in hardness, as the film becomes more organic.
Research and development of carbon nitride film is in progress recently, but the film is still not better in hardness and other properties than conventional carbon-based thin films such as diamond film and diamond-type carbon film. The harder and denser film also requires heating at a temperature of around 1,000° C. and larger discharge power during deposition. However, application of such a method of forming a film at high temperature under high-energy discharge conditions particularly to organic photoreceptors that are vulnerable to heat and discharge is difficult, and thus, the method is impractical.
Accordingly, conventional carbon-based thin films are still insufficient as the surface layer of a photoreceptor, from the points of both hardness and transparency. On the other hand, hydrogenated amorphous silicon carbide films (a-SiC:H) are superior concerning this point. However, such a film often causes image blurring and image deformation due to deposition of discharge products or the like, and thus, it is necessary to use a drum heater for prevention of these problems. In addition, although hydrogenated nitride semiconductors are superior in hardness and transparency, they are also inferior in water resistance and practicability in a high-humidity environment.
In addition to the methods of forming a surface layer in a gas phase described above, methods of forming a surface layer by coating have been proposed. Among them, for improvement in abrasion resistance, use of a polymer compound having siloxane bonds in the surface layer is known. However, a surface layer of such a material is softer than the surface layer formed in a gas phase. Thus, the surface adhesiveness of the layer gradually increases over time, when subjected to abration after damage occurs on the photoreceptor surface, causing the problem of adhesion of toner on the photoreceptor surface resulting in shortening of the life of the photoreceptor.